6502

The 6502 is an 8-bit microprocessor designed by MOS Technology and released in 1975. It became one of the most influential CPUs of the early personal computer era due to its low cost, compact design, and solid performance. The design was led by Chuck Peddle and quickly found widespread use in home computers, game consoles, and embedded systems through licensing to several manufacturers. A notable licensing consequence was the 6501, which was pin-compatible with the Motorola 6800 but led to patent disputes and licensing limitations; the 6502 itself became the dominant variant.

Architecturally, the 6502 uses an 8-bit data bus and a 16-bit address bus, giving it access to up to 64 kilobytes of memory. It exposes three 8-bit registers: accumulator A and index registers X and Y, plus a status register with flags for negative, overflow, decimal, interrupt disable, zero, and carry (with a remaining unused bit). It has an 8-bit stack pointer and a 16-bit program counter. The processor supports multiple addressing modes, including immediate, zero page, absolute, indexed, and indirect, enabling compact and flexible instruction encodings. Instruction execution can take from a few to several clock cycles, depending on mode and operation.

A well-known quirk of the 6502 is the indirect addressing bug used by the JMP (addr) instruction when the address is on a page boundary. The decimal mode flag enables BCD arithmetic, though it is rarely used in modern software. The 6502 spawned a family of related processors and found enduring use in systems such as the Apple II, Atari 8-bit family, Commodore’s 6510-based machines, and in various game consoles (notably with the NES’s Ricoh 2A03/2A07 variants).

Variants and successors, including the CMOS-based 65C02, offered bug fixes and feature tweaks while preserving the core architecture. The 6502’s influence remains evident in the 8-bit computing landscape of the era.